Organics alter hygroscopic behavior of atmospheric particles

TLDR

Atmospheric particle optical and chemical properties, including cloud‑condensation ability, depend on water affinity, and while inorganic salts are known to absorb water proportionally to vapor pressure, similar data for organics are lacking. The study examines whether organics influence atmospheric particle water content by comparing composition and water measurements from Grand Canyon and Los Angeles. Concurrent observations of particle composition and water content were collected at Grand Canyon and Los Angeles to assess organics’ effect on particle hygroscopicity. Organics substantially modify inorganic particle hygroscopicity, enhancing water uptake by 25–40 % at 80–88 % RH in the nonurban Grand Canyon but reducing uptake by 25–35 % at 83–93 % RH in the urban Los Angeles.

Abstract

The optical and chemical properties of atmospheric particles and their ability to act as cloud condensation nuclei (CCN) depend strongly upon their affinity for water. Laboratory experiments have shown that water soluble substances such as ammonium sulfate, ammonium nitrate, and sodium chloride, which are major inorganic components of atmospheric particles, absorb water in an amount proportional to water vapor pressure. Analogous information about the interactions between water and organics, which are another major component of atmospheric particles, is lacking. Here we analyze concurrent observations of particle chemical composition and water content from a continental nonurban (Grand Canyon) and an urban (Los Angeles) location to determine whether the water content of atmospheric particles is influenced by the presence of organics. By comparing the observed water content with the water content expected to be associated with the inorganic fraction, we find that the aggregate hygroscopic properties of inorganic particles are altered substantially when organics are also present. Furthermore, the alterations can be positive or negative. For the nonurban location, organics enhance water absorption by inorganics. In the relative humidity (RH) range of 80–88% organics account for 25–40% of the total water uptake, on average. For the urban location, on the other hand, the net effect of organics is to diminish water absorption of the inorganics by 25–35% in the RH range of 83–93%.